This invention relates to a novel disc brake pad exhibiting a less decrease in the friction coefficient as well as a less wearing at high temperatures and capable of maintaining its adhesion to the metal substrate with little lowering the bond strength during the service life. In particular, the invention relates to a high performance brake which comprises a brake pad free from asbestos reinforcement.
Hitherto, friction materials comprising asbestos as a reinforcement material and a phenol resin as a binder have been employed in a wide range of application, since the materials show an excellent cost performance. However, it was found that asbestos is a carcinogenic substance. When asbestos-containing friction materials are used, the asbestos is worn out and may cause an environmental pollution to happen. This has been a serious problem attracting public attention.
Therefore, it has been studied to use, in place of the asbestos material, any suitable admixture of fibrous materials, such as glass fibers, rock fibers, steel fibers, brass fibers, aramide fibers and the like which are more expensive than asbestos. At first, a brake pad comprising steel fibers as reinforcement material was developed. However, this brake pad shows various disadvantages, for example, vapor lock of the brake fluid caused by the high thermal conductivity of the steel reinforcement material, rusting of the steel base material and fastening thereof to the brake disc with rust; and increased fuel consumption because a big amount of weight by the prior pad is loaded to spring due to the high specific gravity of the prior pad. Thus, this type of brake pads have been used only in a limited range of applications.
On the other hand, it has been required that controlling units such as brake and clutch should be of a high performance and, at the same time should be compact, from a view point of the performance of modern engines (e.g., automotive engines) with respect of their increased power output and improved fuel cost and also from a view point of the designing thereof. For example, it is required for disc brake units that they are operable at higher temperatures than those at which the conventional disc brake units have been operated. For example, it is desired that the units are operable at temperatures of higher than 300.degree. C. or even 350.degree. C. It is also required from the safety aspect that the performance of the friction materials in the disc brake units should not degrade at high temperatures and should provide an elongated service life.
However, the conventional asbestos/phenol resin-based materials have not been able to satisfy these requirements.
It has been proposed to employ a reinforcement material comprising any suitable admixture of glass fibers, rock wool, steel fibers, brass fibers, aramide fibers and the like and a binder system comprising a phenol resin and a melamine resin. With this proposition, although the smoking problem experienced with the conventional materials has been improved, the wear-resistant properties have not been improved.
There is disclosed in Japanese Patent Public Disclosure (KOKAI) No. 63-10658 assigned to the assignee of this application a method for producing wear resistant, sliding parts comprising the steps of preparing a curable resin composition comprising a cyanate ester resin and finely milled carbon fibers and shaping said curable resin composition on the surface of a metallic substrate to give an integrated cured product. In this Japanese KOKAI specification, it is said that the product shows a low friction coefficient. On the contrary the friction materials used in the disc brake units should maintain an appropriately controlled level of friction coefficient in the temperature range of up to 300.degree. C. or even more and be well resistant to wearing under the operation conditions. The requisites for the disc brake friction materials are inconsistent with the low friction coefficient property of the product disclosed in the above-mentioned Japanese KOKAI specification. Thus, it is far from the teaching of the Japanese KOKAl specification to expect that a cyanate ester based-resin composition is applicable in the field of manufacturing of disc brakes having an appropriately controlled high level of friction coefficient.
We have intensively studied to solve the above discussed problems of the prior art and found surprisingly that when a disc brake pad is produced using a cyanate ester based curable resin composition as a binder, the pad is capable of well maintaining a satisfactory level of friction coefficient even at such elevated temperatures (exceeding 350.degree. C.) that otherwise the composition itself would not be able to resist in air. It has been also found that the pad shows an acceptably low wearing even at the elevated temperatures. Further, it has been found that the pad formed integrally with a metallic substrate used for the disc brake fabrication has an excellent bond strength to the substrate at highly elevated temperatures. These findings have resulted in the present invention.